filemanager/filepicker treeview displays the wrong image y/ln.png instead of y/ln.gif

If you view the folder, or open filemanager in 'treeview' mode there are now wrong images.
Instead of lines on the left from the folder there is folder icon.

theme/base/style/filemanager.css says:

/* last sibling, no children */

.file-picker .ygtvln, .filemanager .ygtvln {background: url('[[pix:moodle|y/ln]]') 0px 0px no-repeat;width:17px;height:22px;}

.dir-rtl .file-picker .ygtvln {background: url('[[pix:moodle|y/ln_rtl]]') 0px 0px no-repeat;width:17px;height:22px;}

for some reason we have files pix/y/ln.gif and pix/y/ln.png that are completely different.
Now .png is used which is not a line but a folder

(tested only on master but may be the same issue in previous versions)